Ablation system and method of use

ABSTRACT

A system and method for creating lesions and assessing their completeness or transmurality. Assessment of transmurality of a lesion is accomplished by monitoring the depolarization signal in a local electrogram taken using electrodes located adjacent the tissue to be ablated. Following onset of application of ablation energy to heart tissue, the local electrogram is measured with electrodes located adjacent tissue to be ablated so that the ablation energy to ablation elements can be selectively reduced or terminated when transmurality is detected.

RELATED US APPLICATION DATA

This application is a continuation of U.S. Ser. No. 10/685,236, filed.Oct. 14, 2003 by Francischelli et al., now U.S. Pat. No. 7,029,470,which is a continuation of U.S. Ser. No. 10/132,392, filed Apr. 24, 2002by Francischelli et al., now U.S. Pat. No. 6,663,627, and also claimsthe benefit of Provisional U.S. Patent Application No. 60/236,923, filedApr. 26, 2001 by Francischelli et al., incorporated herein by referencein its entirety.

BACKGROUND OF THE INVENTION

The present invention relates to tissue ablation devices generally andrelates more particularly to devices adapted to ablate lines of tissue,for example for use in conjunction with an electrosurgical version ofthe Maze procedure.

The Maze procedure is a surgical intervention for patients with chronicatrial fibrillation (AF) that is resistant to other medical treatments.The operation employs incisions in the right and left atria which dividethe atria into electrically isolated portions which in turn results inan orderly passage of the depolarization wave front from the sino-atrialnode (SA Node) to the atrial-ventricular node (AV Node) while preventingreentrant wave front propagation. Although successful in treating AF,the surgical Maze procedure is quite complex and is currently performedby a limited number of highly skilled cardiac surgeons in conjunctionwith other open-heart procedures. As a result of the complexities of thesurgical procedure, there has been an increased level of interest inprocedures employing electrosurgical devices or other types of ablationdevices, e.g. thermal ablation, micro-wave ablation, cryo-ablation orthe like to ablate tissue along pathways approximating the incisions ofthe Maze procedure. Electrosurgical systems for performing suchprocedures are described in U.S. Pat. No. 5,916,213, issued toHiassaguerre, et al. U.S. Pat. No. 5,957,961, issued to Maguire, et al.and U.S. Pat. No. 5,690,661, all incorporated herein by reference intheir entireties. Cryo-ablation systems for performing such proceduresare described in U.S. Pat. No. 5,733,280 issued to Avitall, alsoincorporated herein by reference in its entirety.

In conjunction with the use of electrosurgical ablation devices, variouscontrol mechanisms have been developed to control delivery of ablationenergy to achieve the desired result of ablation, i.e. killing of cellsat the ablation site while leaving the basic structure of the organ tobe ablated intact. Such control systems include measurement oftemperature and impedance at or adjacent to the ablation site, as aredisclosed in U.S. Pat. No. 5,540,681, issued to Struhl, et al.,incorporated herein by reference in its entirety.

Additionally, there has been substantial work done toward assuring thatthe ablation procedure is complete, i.e. that the ablation extendsthrough the thickness of the tissue to be ablated, before terminatingapplication of ablation energy. This desired result is some timesreferred to as a “transmural” ablation. For example, detection of adesired drop in electrical impedance at the electrode site as anindicator of transmurality is disclosed in U.S. Pat. No. 5,562,721issued to Marchlinski et al, incorporated herein by reference in itsentirety. Alternatively, detection of an impedance rise or an impedancerise following an impedance fall are disclosed in U.S. Pat. No.5,558,671 issued to Yates and U.S. Pat. No. 5,540,684 issued to Hassler,respectively, also incorporated herein by reference in their entireties.Because ablated heart tissue is necrotic, it does not depolarize andtherefore does not contribute to the depolarization signal. This facthas recently led some physicians to use the amplitude of a locallyacquired electrogram signal to determine whether a lesion is complete.For example, during provision of R-F energy at a constant power level,the physician may monitor the amplitude of electrograms obtained usingelectrodes adjacent the ablation site, and, in response to a defineddrop, e.g. 75%, may terminate provision of RF energy.

Three basic approaches have been employed to create elongated lesionsusing electrosurgical devices. The first approach is simply to create aseries of short lesions using a contact electrode, moving it along thesurface of the organ wall to be ablated to create a linear lesion. Thiscan be accomplished either by making a series of lesions, moving theelectrode between lesions or by dragging the electrode along the surfaceof the organ to be ablated and continuously applying ablation energy, asdescribed in U.S. Pat. No. 5,897,533 issued to Mulier, et al.,incorporated herein by reference in its entirety. The second basicapproach to creation of elongated lesions is simply to employ anelongated electrode, and to place the elongated electrode along thedesired line of lesion along the tissue. This approach is described inU.S. Pat. No. 5,916,213, cited above and. The third basic approach tocreation of elongated lesions is to provide a series of electrodes andarrange the series of electrodes along the desired line of lesion. Theelectrodes may be activated individually or in sequence, as disclosed inU.S. Pat. No. 5,957,961, also cited above. In the case ofmulti-electrode devices, individual feedback regulation of ablatedenergy applied via the electrodes may also be employed. The presentinvention is believed useful in conjunction with all three approaches

SUMMARY OF THE INVENTION

The present invention is directed toward an improved system for creatinglesions and assessing their completeness or transmurality. In thepreferred embodiment as disclosed, the apparatus for producing thelesions is an electrosurgical device, in particular a saline-irrigatedbipolar electrosurgical forceps. However, the mechanism for assessinglesion transmurality provided by the present invention is believeduseful in other contexts, including unipolar R-F ablation and R-Fablation using catheters or hand-held probes. The mechanism forassessing transmurality may also be of value in the context of othertypes of ablation systems, including those in which ablation occurs inconjunction with an induced rise in tissue temperature, such as thoseapplying ablation energy in the form of microwave radiation, light(laser ablation) or heat (thermal ablation). The invention may also beuseful in conjunction with other types of ablation, includingcryo-ablation, ultrasound ablation and chemical ablation.

According to the present invention, assessment of transmurality of alesion is accomplished by monitoring the depolarization signal amplitudein a local electrogram taken using electrodes located adjacent thetissue to be ablated. In the context of R-F ablation, measurement ofelectrogram amplitude may be done using the ablation electrodes or maybe done using dedicated electrodes adjacent to the ablation electrodes.In the context of the other types of ablation discussed above,electrogram measurement would typically be accomplished by means of adedicated set of measurement electrodes.

Following onset of application of ablation energy to heart tissue, theamplitude of a local electrogram measured with electrodes locatedadjacent tissue to be ablated first gradually drops and then stabilizes,indicating that the tissue being monitored has ceased making anycontribution to the sensed electrogram. The amplitude drop (ΔEGM) or thefollowing amplitude plateau “P” may be used alone or together asindicators of transmurality employed by the present invention. Theamplitude drop may be compared to a pre-set value (ΔEGM ? a). (Theplateau “P” may be detected in response to a determination that the rateof amplitude change is less than a defined value over a series ofamplitude measurements or over a defined duration (|dA/dt|=b). In someembodiments, detection of a rapid drop in amplitude (dA/dt=d) may beemployed as an indicator that the ablation process is proceeding tooquickly and may be employed to trigger a reduction in the power ofapplied ablation energy. In other embodiments, detection of aninsufficiently rapid drop in amplitude (dA/dt=d) may be employed as anindicator that the ablation process is proceeding too slowly and may beemployed to trigger an increase in the power of applied ablation energy.

In the context of R-F ablation, the invention is believed valuable inconjunction with an ablation device having multiple, individuallyactivatable electrodes or electrode pairs to be arranged along a desiredline of lesion. In this context, the mechanism for determiningtransmurality of lesions adjacent individual electrodes or pairs may beused to deactivate individual electrodes or electrode pairs, when thelesions in tissue adjacent these individual electrodes or electrodepairs are complete. This allows the creation of an essentially uniformlesion along the line of electrodes or electrode pairs, regardless ofdifferences in tissue thickness adjacent the individual electrodes orelectrode pairs. The invention is also believed useful in conjunctionwith assessment of transmurality of lesions produced by devices havingonly a single electrode or single electrode pair. Similar considerationsapply to the use of the present invention in the contexts of other typesof ablation as listed above.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of a type of electrosurgical hemostat that may beused in conjunction with the present invention.

FIGS. 2 a and 2 b illustrate alternative electrode configurations for ahemostat generally according to FIG. 1.

FIG. 3 illustrates the fall and plateau of electrogram amplitudemeasured using electrodes adjacent tissue during ablation.

FIG. 4 is a functional block diagram of an RF generator appropriate foruse in practicing the present invention, particularly adapted for use inconjunction with an ablation system employing multiple, individuallyactivatable electrodes or electrode pairs.

FIG. 5 is a functional flow chart illustrating a first mode of operationof the device illustrated in FIG. 4 in practicing the present invention.

FIG. 6 is a functional flow chart illustrating a first alternative modeof operation of a device as in FIG. 4 in practicing the presentinvention.

FIG. 7 is a functional flow chart illustrating a modification to themode of operation according to FIG. 6.

FIG. 8 is a functional flow chart illustrating a second alternative modeof operation of a device as in FIG. 4 in practicing the presentinvention.

FIG. 9A is a functional flow chart illustrating a modification to themodes of operation according to FIGS. 5, 6 and 8.

FIG. 9B is a functional flow chart illustrating an additionalmodification to the modes of operation according to FIGS. 5, 6 and 8.

FIG. 10 is a functional flow chart illustrating a first method ofoperation of a device as in FIG. 4 to activate and deactivate individualelectrodes or electrode pairs.

FIG. 11 is a functional flow chart illustrating a second method ofoperation of a device as in FIG. 4 to activate and deactivate individualelectrodes or electrode pairs.

DETAILED DESCRIPTION PREFERRED EMBODIMENTS

FIG. 1 is a plan view of a bipolar, saline irrigated electrosurgicalhemostat of a type that may be employed in conjunction with the presentinvention. The hemostat is provided with elongated handles 11 and 12 anda lock mechanism 14, similar to a conventional surgical hemostat. Thehandles are connected to one another by pivot or hinge 16, and continuedistally in the form of elongated jaws 18 and 19. Jaws 18 and 19 carryan elongated electrode or series of electrodes 24, 25, respectively, towhich ablation energy, e.g. RF energy is applied by means of conductors21 and 22. The electrodes are adapted to be irrigated by a salinesolution or other conductive fluid along their length, provided viainlet tubes 20 and 23. In operation, tissue to be ablated is compressedbetween the jaws, and RF energy is applied between the electrode orelectrode sets 24 and 25, as generally described in U.S. Pat. No.6,096,037 issued to Mulier et al incorporated herein by reference in itsentirety.

FIG. 2 a shows a first embodiment of an electrode arrangement for ahemostat generally as illustrated in FIG. 1. Illustrated componentscorrespond to identically numbered components in FIG. 1. In thisembodiment, electrodes 24 and 25 take the form of elongated coilelectrodes 30 and 32, mounted around porous tubes 34 and 36, throughwhich saline or other conductive fluid is delivered. The arrangement ofthe electrodes may also be reversed, for example placing coils 30 and 32within elongated porous tubes 34 and 36, to accomplish a similar result.Alternatively, any other arrangement for providing an elongatedelectrode and delivery of saline solution along the length thereof maybe substituted. The particular configuration of the electrode is notcritical to the present invention. For example, irrigated electrodescorresponding to those described in U.S. Pat. No. 6,096,037 issued toMulier, et al., U.S. Pat. No. 5,876,398 issued to Mulier, et al., U.S.Pat. No. 6,017,378 issued to Brucker, et al or U.S. Pat. No. 5,913,856issued to Chia, et al., all incorporated herein by reference in theirentireties may also be substituted. It should also be noted that whilethe electrode system as illustrated in FIG. 2 a is a bipolar system, theinvention may also be employed in conjunction with unipolar electrodesand/or in the form of a probe or a catheter. In some embodiments,irrigation of the electrodes may be omitted.

FIG. 2 b illustrates an alternative embodiment of an electrode systemfor a hemostat generally as illustrated in FIG. 1. In this case, ratherthan a single pair of electrodes, multiple electrode pairs are provided.The electrode pairs comprise coil electrodes 40 and 42, 44 and 46, 48and 50, 52 and 54, and 56 and 58. However, other pairings of electrodesmight also be substituted, for example, pairing electrodes 40 and 44,electrodes 48 and 52 or the like. In this embodiment, the electrodepairs are mounted around porous plastic tubes 60 and 62 through whichsaline or other electrically conductive fluid is delivered. As in thecase with the embodiment of FIG. 2 a, the arrangement of theseelectrodes may readily be reversed, placing the electrodes within thelumen of plastic tube 60 or 62 and any other arrangement providingmultiple, irrigated electrodes may also be substituted. As in the caseof the embodiment of FIG. 2 a, unipolar electrodes might be substitutedfor the multiple bipolar pairs as illustrated and/or the invention maybe practiced in conjunction with a multi-electrode probe or catheter.

In use, the hemostat is arranged so that the tissue to be ablated islocated between the jaws 18 and 19, and pressure is applied in order tocompress the tissue slightly between the jaws to ensure good electricalcontact. All electrode pairs may be activated individually and may beindividually deactivated when the lesions between the individualelectrode pairs are completely transmural. Alternatively, electrodepairs could be activated sequentially, with one pair deactivated upon adetection of a complete lesion between the electrode pair, followed byactivation of the next sequential electrode pair. Corresponding use ofthe invention in conjunction with a series of unipolar electrodes, forexample corresponding to electrodes along one of the two jaws inconjunction with a remote ground plate or a similar series ofindividually activatable electrodes on a catheter or probe inconjunction with a ground plate is also possible.

FIG. 3 is a graph illustrating measured local electrogram amplitude “A”vs. time across tissue located between the electrodes of an irrigatedbipolar hemostat as illustrated in FIG. 1. FIG. 3 illustrates the dropin electrogram amplitude followed by an amplitude plateau. The amplitudedrop (ΔEGM) or the following amplitude plateau “P” may be used alone ortogether as indicators of transmurality employed by the presentinvention. In some embodiments, detection of a rapid rate of drop inamplitude (dA/dT) may be employed as an indicator that the ablationprocess is proceeding too quickly and may be employed to trigger areduction in the power of applied ablation energy. In other embodiments,detection of an insufficient rate of drop in amplitude (dA/dT) may beemployed as an indicator that the ablation process is proceeding tooslowly and may be employed to trigger an increase in the power ofapplied ablation energy.

FIG. 4 is a functional block diagram illustrating one embodiment of anR-F generator system for use in conjunction with the present invention.In this embodiment, separately controllable R-F outputs are provided forindividual ablation electrodes or electrode pairs on an associated R-Fablation device, for example as in FIG. 2B. The R-F generator could ofcourse also be used with ablation devices having only a single electrodeor electrode pair as in FIG. 2A. With the exception of the electrogramamplitude measurement circuits discussed below, the generatorcorresponds generally to that described in conjunction with FIG. 16 ofthe '961 patent issued to Maguire, et al., cited above. The RF generatordisclosed in the '961 patent provides feedback control of R-F powerbased upon either measured power (constant power) or measuredtemperature. The present invention is somewhat easier to implement inconjunction with the constant power mode, but may also be adapted to atemperature-regulated mode or to other feedback power regulationmechanism.

Display 804 and controls 802 are connected to a digital microprocessor800, which permits interface between the user and the remainder of theelectrical components of the system. Microprocessor 800 operates undercontrol of stored programming defining its operation includingprogramming controlling its operation according to the presentinvention, as discussed in more detail below. Microprocessor 800provides control outputs to and receives input signals from theremaining circuitry via address/data bus 806. In particular, themicroprocessor 800 provides for monitoring of power, current, voltage,electrogram amplitude and temperature. As necessary, the microprocessorwill provide this information to the display 804. Additionally, themicroprocessor 800 permits the user to select the control mode (eithertemperature or power) and to input the power set point, temperature setpoint, and a timer set point to the system. The primary source of powerfor the radio-frequency generator may be a 12 V battery rated at 7.2ampere-hours or the device may be AC powered. A back-up battery (notshown) such as a lithium cell may also be provided to provide sufficientpower to the microprocessor 800 to maintain desired memory functionswhen the main power is shut off.

The power supply system as illustrated includes a desired number “M” ofindividually controllable R-F power supplies and receives temperatureinputs from a desired number “N” of temperature sensing devices in theablation device, illustrated schematically at 838 and receiveselectrogram amplitude inputs from a desired number “M” of electrogrammonitoring circuits. Each R-F power supply includes a transformer (822,824, 826), a power control circuit (810, 812, 814) and a powermeasurement circuit (816, 818, 820). A crystal-locked radio-frequencyoscillator 840 generates the switching pulses, which drive both thepower transformers (822, 824, 826) and the power controllers (810, 812,814). Power controllers (810, 812, 814) may be analog controllers whichoperate by pulse-width modulation by comparing a power set point signalfrom microprocessor 800 with an actual power signal generated by a powermeasurement circuit (816, 818, 820), which may, for example, includea.torroidal transformer coupled to the power output from the associatedtransformer (822, 824, 826). The power measurement circuits (816, 818,820) multiply the output current and voltage and provide the resultingactual power signal to both the power controllers (810, 812, 814) andthe microprocessor 800.

The R F power output of the transformers (822, 824, 826) is provided tointerface board 808, and thereby is provided to the ablation electrodeor electrodes on the ablation device 838. Separate analog comparatorcircuits (not illustrated) may also be provided for monitoring theoutput of the power measurement circuits (816, 818, 820), in order toshut-off current to the output transformers (822, 824, 826) if the powerexceeds a limit, typically 55 watts. Power transformers (822, 824, 826)may include center taps, which receive the outputs of the powercontrollers (810, 812, 814). Secondary windings of the transformers(822, 824, 826) may provide for continuous monitoring of the appliedvoltage in order to permit the power calculations by power measurementcircuits (816, 818, 820).

The illustrated power R-F generator system employs software controlledtemperature processing, accomplished by micro processor 800, whichreceives the “N” temperature input signals from temperature measurementcircuits (828, 830, 832), each of which are coupled to a correspondingtemperature sensor in ablation device 838 by means of an electricalconnector, illustrated schematically at 836 and interface circuit 808.If programmed to operate in the temperature controlled mode, processor800 receives the “N” temperature signals and, based upon the indicatedtemperatures, defines power set points for each of the power control.circuits (810, 812, 814), which in the manner described above controlthe power levels applied to electrodes on the catheter through interface834. Processor 800 may also selectively enable or disable any of the “M”provided R-F power supplies, in response to external control signalsfrom controls 802 or in response to detected anomalous temperatureconditions.

In addition to the circuitry as described above and disclosed in theMaguire, et al. '961patent, the apparatus of FIG. 4 includes multipleelectrogram monitoring circuits EGM1, EGM2 . . . EGMM (843, 845 and 847respectively), which may include one or more peak detectors coupled tosample and hold circuits, operating generally as described in U.S. Pat.No. 6, 266,566 issued to Nichols, et al., U.S. Pat. No. 6,029,986 issuedto Kim, et al., U.S. Pat. No. 6,095,150 issued to Panescue, et al. orU.S. Pat. No. 5,685,315, issued to McClure, et al. , also allincorporated herein by reference in their entireties. The electrogrammonitoring circuits measure electrogram amplitudes sensed usingelectrodes on the RF ablation device. Measured amplitudes may bepeak-to-peak measurements of depolarization wave amplitudes or absolutepeak value measurements, positive peak vales or negative peak values.Amplitude measurements employed in practicing the invention may beindividually measured values or digitally filtered values obtained byaveraging a series of individually measured amplitudes. The electrogramsignals from the electrodes on the ablation device 838 may first befiltered through low-pass filters Fl, F2, . . . FN (842, 844, 846) andmay be measured between the ablation electrodes or between electrodeslocated adjacent the ablation electrodes. Measurements are preferablymade during interruptions in the delivery of ablation energy to theablation electrodes, to minimize noise-sensing problems. Optionally, anEGM trigger circuit 850, coupled to surface EGM electrodes 852 and 854,may trigger electrogram measurement.

Individual electrogram amplitude measurements made by measurementcircuits 843, 845 and 847 are provided to the address/data bus 806 andthence to microprocessor 800 for analysis to determine whether thebehavior of the measured electrogram amplitude over time, indicates thatthe lesion associated with the measured amplitudes is completelytransmural. As discussed in more detail below, a determination oftransmurality may be made in response to detection of a defined drop inelectrogram amplitude and/or a series of amplitude measurements that arerelatively constant, over a desired period of time or over a definednumber of successive amplitude measurements. In some embodiments, anabrupt drop in electrogram amplitude may also be employed to reduce thepower level of ablation energy delivered to the tissue being monitored.

In cases in which an alternative ablation energy generation apparatus isemployed, particularly those in which a rise in tissue temperature isinduced, e.g. laser, microwave or thermal ablation, the R-F generationcircuitry of FIG. 4 would be replaced with a corresponding alternativeablation energy generation apparatus. The measurement of electrogramamplitude and its use according to the present invention, however, maystill be useful in conjunction with these alternative ablation energygeneration systems. Similarly, he measurement of electrogram amplitudeand its use according to the present invention may also be useful inconjunction with other forms of ablation such as cryo-ablation,ultrasound ablation and chemical ablation.

FIG. 5 is a functional flow chart illustrating the operation of a deviceas in FIG. 4, according to the present invention. The flow chart of FIG.5 illustrates operation of the device of FIG. 4 to control provision ofR-F energy to an individual electrode or electrode pair. In the eventthat multiple electrodes or electrode pairs are employed, the controlmethodology of FIG. 5 would be applied to each electrode or electrodepair individually, as discussed in more detail below in conjunction withFIGS. 10 and 11.

The flow chart of FIG. 5 illustrates a method of assessing transmuralityand terminating delivery of ablation energy to an electrode or anelectrode pair responsive to detection of a specified drop inelectrogram amplitude, e.g. a 75% drop. Following the detection ofrequired amplitude drop, the device may wait a defined time period toassure completion of the lesion and then terminate the application ofablation energy to the associated electrode pair. Alternatively,termination of application of ablation energy may occur concurrent withdetection of the required amplitude drop. Measurement of electrogramamplitude in tissue adjacent with the ablation electrode or electrodepair electrode pair may be made using the ablation electrodes themselvesor using electrodes located in proximity to the ablation electrodes, forexample corresponding to those used to measure impedance in thedescribed in Yates '671 patent, incorporated by reference above.

After initialization at 200, the microprocessor 800 (FIG. 4) causes theelectrogram measurement circuitry associated with the electrode orelectrode pair being evaluated to acquire a base line or initialamplitude value EGM_(i) at 202. The microprocessor then beginsapplication of ablation energy to the monitored tissue at 204. Duringapplication of ablation energy, at defined intervals or in response totrigger signals from an EGM trigger circuit (850, FIG. 4) themicroprocessor obtains and stores electrogram measurements at 206.Delivery of ablation energy may be interrupted during the electrogrammeasurement period. With each obtained amplitude measurement, theprocessor determines whether the required amplitude drop has occurred.This determination may be made, for example, in response to the firstamplitude measurement below a preset required drop value “a”, e.g. −75%,to a series of a required number of measured amplitudes below therequired drop value, e.g. 2 or 3 measurements, or to a requiredproportion of measured amplitudes below the required drop value, e.g. 2of 3 measurements. Alternatively, as noted above, averaged amplitudesmay be calculated with each measurement, in which case a drop in theaveraged amplitude may be employed to detect transmurality. Ablation andelectrogram measurement continues until the required drop has beendetected at 208. The termination of application of ablation energy tothe tissue being monitored then occurs at 210. The termination ofablation may occur concurrent with detection of the required drop or apreset delay thereafter to assure complete transmurality.

FIG. 6 is a functional flow chart illustrating the operation of a deviceas in FIG. 4, according to a second embodiment of the present invention.The flow chart of FIG. 6 illustrates operation of the device of FIG. 4to control provision of R-F energy to an individual electrode orelectrode pair. In the event that multiple electrodes or electrode pairsare employed, the control methodology of FIG. 6 would be applied to eachelectrode or electrode pair individually, as discussed in more detailbelow in conjunction with FIGS. 10 and 11.

The flow chart of FIG. 6 illustrates a method of assessing transmuralityand terminating delivery of ablation energy to an electrode or anelectrode pair responsive to detection of a specified drop inelectrogram amplitude, e.g. a 75% drop in conjunction with detection ofan electrogram amplitude plateau. Following the detection of therequired amplitude drop and plateau, the device may wait a defined timeperiod to assure completion of the lesion and then terminate theapplication of ablation energy to the associated electrode pair.Alternatively, termination of application of ablation energy may occurconcurrent with detection of the required amplitude drop and plateau.Measurement of electrogram amplitude in tissue adjacent with theablation electrode or electrode pair electrode pair may be made usingthe ablation electrodes themselves or using electrodes located inproximity to the ablation electrodes.

After initialization at 300, the microprocessor 800 (FIG. 4) causes theelectrogram measurement circuitry associated with the electrode orelectrode pair being evaluated to acquire a baseline or initialamplitude value EGM_(i) at 302. The microprocessor then beginsapplication of ablation energy to the monitored tissue at 304. Duringapplication of ablation energy, the microprocessor obtains and storeselectrogram measurements at 306, as discussed above in conjunction withFIG. 5. With each obtained amplitude measurement, the processordetermines whether the required amplitude drop has occurred at 308. Thisdetermination may be made as described above in conjunction with FIG. 5.

If the required drop is detected at 308, at 310, the microprocessor 800employs the stored electrogram amplitude measurements to calculatedA/dt, which may, for example, be calculated based on net variation ofelectrogram amplitude over a series of 2 or 3 measurements. As discussedabove in conjunction with detection of the required amplitude drop,averaged amplitude values may also be used to calculate dA/dt. Theabsolute value of dA/dt, i.e., |dA/dt| may employed to assess whether ornot an electrogram amplitude plateau has been reached at 310, forexample by verifying that a series of values of |dA/dt| are all (e.g. 3of 3) or predominantly (e.g. 2 of 3) below a defined variability value“b”.

The processor continues to collect amplitude measurements and makecalculations until such time as an amplitude plateau is recognized at310 and a sufficient amplitude drop is recognized at 308. When both ofthese criteria have been met, the termination of application of ablationenergy to the tissue being monitored then occurs at 312. The terminationof ablation may occur concurrent with detection of the required drop ora preset delay thereafter to assure complete transmurality.

FIG. 7 is a functional flow chart illustrating an optional additionalset of operations for implementing a transmurality measurement methodgenerally as in FIG. 6. The additional operations provide for anincrease in ablation energy responsive to detection of an electrogramamplitude plateau at 310 (FIG. 6), in order to verify that ablation iscomplete. The processor triggers an increase in power at 420, forexample 5 to 25%, and starts time period of a few seconds at 422.Measurement and calculation of electrogram amplitudes continues at 424until either a significant drop in amplitude is detected at 426 or thetimer expires at 428. If a significant drop is detected, for example thesame magnitude of drop that would have prevented plateau detection at310 (FIG. 6), the processor re-initiates the process of detecting aplateau at 306 (FIG. 6). If the time period expires with no furthersignificant drop in electrogram amplitude, the processor terminates theablation process at 312 (FIG. 6).

FIG. 8 is a functional flow chart illustrating the operation of a deviceas in FIG. 4, according to an additional alternative embodiment of thepresent invention. The flow chart of FIG. 8 illustrates operation of thedevice of FIG. 4 to control provision of R-F energy to an individualelectrode or electrode pair. In the event that multiple electrodes orelectrode pairs are employed, the control methodology of FIG. 8 would beapplied to each electrode or electrode pair individually, as discussedin more detail below in conjunction with FIGS. 10 and 11.

The flow chart of FIG. 8 illustrates a method of assessing transmuralityand terminating delivery of ablation energy to an electrode or anelectrode pair responsive to detection of an electrogram amplitudeplateau in conjunction with a defined minimum ablation duration.Following the detection of the required plateau, the device may wait adefined time period to assure completion of the lesion and thenterminate the application of ablation energy to the associated electrodepair. Alternatively, termination of application of ablation energy mayoccur concurrent with detection of the required amplitude drop andplateau. Measurement of electrogram amplitude in tissue adjacent withthe ablation electrode or electrode pair electrode pair may be madeusing the ablation electrodes themselves or using electrodes located inproximity to the ablation electrodes.

After initialization at 400, the microprocessor 800 (FIG. 4) causes theelectrogram measurement circuitry associated with the electrode orelectrode pair being evaluated to acquire a base line or initialamplitude value EGM_(i) at 402. The microprocessor then starts aduration time interval at 404 and begins application of ablation energyto the monitored tissue at 406. During application of ablation energy,the microprocessor obtains and stores electrogram measurements at 408,as discussed above in conjunction with FIG. 5. With each obtainedamplitude measurement, the processor 800 determines whether an amplitudeplateau has occurred at 410, in the manner discussed above inconjunction with FIG. 6. If a plateau is detected, the processor checkto see if the required minimum ablation time, e.g. 10 seconds, haselapsed at 412.

The processor continues to collect amplitude measurements until suchtime as an amplitude plateau is recognized at 410 and a sufficient timehas elapsed at 412. When both of these criteria have been met, thetermination of application of ablation energy to the tissue beingmonitored then occurs at 414. The termination of ablation may occurconcurrent with detection of the required drop or a preset delaythereafter to assure complete transmurality.

FIG. 9A illustrates an additional set of operations for implementing atransmurality measurement method generally as in FIG. 5, 6 or 8. Theoperations of FIG. 8 may be performed following the measurement ofelectrogram amplitude at 206, 306 or 408 (FIGS. 5, 6 and 8). In theadditional operations illustrated in FIG. 9A, the microprocessor checksat 500 to determine whether electrogram amplitude is decreasing toorapidly, for example in response to dA/dt having a value less than adefined negative threshold “d”, e.g. −10 mv/s. In response to a detectedexcessive decrease at 500, the processor reduces the power level of theablation energy being applied to the monitored tissue at 502 to slow theablation process. Operation of the device then continues as in FIG. 5, 6or 8.

FIG. 9B also illustrates an additional set of operations forimplementing a transmurality measurement method generally as in FIG. 5,6 or 8. The operations of FIG. 8 may be performed following themeasurement of electrogram amplitude at 206, 306 or 408 (FIGS. 5, 6 and8) and may be performed in conjunction with or instead of the operationsof FIG. 9A. In the additional operations illustrated in FIG. 9B, themicroprocessor checks at 510 to determine whether electrogram amplitudeis decreasing too rapidly, for example in response to dA/dt having avalue greater than a defined negative threshold “e”, e.g. −1 mv/s. Inresponse to a detected insufficient decrease at 510, the processorincreases the power level of the ablation energy being applied to themonitored tissue at 512 to speed the ablation process. Operation of thedevice then continues as in FIG. 5, 6 or 8.

FIG. 10 is a functional flow chart illustrating the over-all operationof the device in conjunction with a multi electrode or multi electrodepair ablation apparatus. In the flow chart of FIG. 10, all the ablationelectrodes or electrode pairs are activated simultaneously andindividual ablation electrodes or electrode pairs are deactivated inresponse to electrogram amplitude measurements associated with theelectrode pair indicating that the lesion formed between that electrodepair is completely transmural. In this circumstance, the ablation systemworks as follows.

After initialization at 600, all electrodes 1-X are activated at 602,meaning that ablation energy is provided to all electrodes and electrodepairs. The microprocessor measures the electrogram amplitude associatedwith a first electrode or electrode pair at 604 and then at 608 checksto see whether transmurality criteria are met for a first ablationelectrode or electrode pair at 604, using the criteria discussed abovein conjunction with any of FIGS. 5-8. If so, the ablation electrode orelectrode pair is deactivated at 610 by ceasing the delivery of ablationenergy to the electrode or electrode pair. If not, the microprocessormeasures the electrogram amplitude associated with the next ablationelectrode or electrode pair at 606 and checks transmurality criteria forthe next electrode at 608. This process continues until all electrodesare deactivated at 612, after which the procedure is deemed complete at614 and the ablation process is terminated at 616.

FIG. 11 illustrates a functional flow chart of overall operation of adevice in which a multi-electrode or multi-electrode pair ablationapparatus is employed, as in FIG. 10. In this embodiment, however,ablation electrodes or electrode pairs are activated sequentially. Afterinitialization at 700, the microprocessor activates delivery of ablationenergy to the first ablation electrode or electrode pair at 702 andmeasures electrogram amplitude at 704. At 708, the processor and, in thesame manner as described for FIG. 9 above, checks to determine whethertransmurality criteria have been met. If so, the ablation electrode orelectrode pair is deactivated at 710. If not, application of ablationenergy continues until the transmurality criteria are met as describedabove. After deactivation of an electrode or electrode pair at 710, themicroprocessor checks to determine whether all electrodes have beenactivated and deactivated at 712, if not, the microprocessor thenactivates the next electrode or electrode pair at 706 and initiatesdelivery of ablation energy to that electrode or electrode pair. Thisprocess continues until the last electrode has been deactivated at 712,following which the microprocessor determines that the ablation processis complete at 714 and the ablation process is stopped at 716.

The overall operational methodology of FIG. 10 is believed to bedesirable in that it allows for a more rapid completion of an ablationprocedure. However, the overall operational method is described in FIG.11 has the advantage that it may allow the use of a somewhat simplifiedgenerator because that multiple, separate electrogram measurementcircuits, power control circuits, and the like need not be provided foreach electrode or electrode pair. A simple switching mechanism may beused in conjunction with only a single RF generator and electrogrammeasurement circuit to successively apply energy to each electrode andto monitor electrogram amplitude according to the invention.

1. An ablation system, comprising: a generator for generating ablationenergy; an ablation device comprising an ablation element operablycoupled to the generator and locatable adjacent a tissue site to beablated, for applying ablation energy to the tissue site; an electrogrammeasurement element mounted to the ablation device so that theelectrogram measurement element is adjacent the tissue site when theablation element is adjacent the tissue site; electrogram amplitudemeasurement circuitry operably coupled to the electrogram measurementelement to measure electrogram amplitude at the tissue site, using theelectrogram measurement element; and control circuitry operably coupledto the generator to initiate and terminate the application of ablationenergy to the ablation element, wherein the control circuitry isoperably coupled to the electrogram amplitude measurement circuit andterminates application of ablation energy to the ablation elementresponsive to occurrence of a plateau in the electrogram amplitudemeasured by the electrogram amplitude measurement circuitry followinginitiation of application of ablation energy to the ablating element. 2.The system of claim 1 wherein the ablation element is an ablationelectrode.
 3. The system of claim 2 wherein the ablation electrode is anirrigated ablation electrode.
 4. The system of claim 2 wherein theablation electrode is employed as the electrogram measurement element.5. The system of claim 1 wherein the electrogram measurement element isan electrogram measurement electrode.
 6. The system of claim 1 whereinthe generator is an R-F generator.
 7. The system of claim 1 wherein theablation energy is R-F energy.
 8. The system of claim 1 wherein theablation energy is microwave radiation.
 9. The system of claim 1 whereinthe ablation energy is laser energy.
 10. The system of claim 1 whereinthe ablation energy is heat energy.
 11. The system of claim 1 whereinthe ablation energy is ultrasound energy.
 12. An ablation system,comprising: a generator for generating ablation energy; an ablationdevice comprising an ablation element operably coupled to the generatorand locatable adjacent a tissue site to be ablated, for applyingablation energy to the tissue site; an electrogram measurement elementmounted to the ablation device so that the electrogram measurementelement is adjacent the tissue site when the ablation element isadjacent the tissue site; electrogram amplitude measurement circuitryoperably coupled to the electrogram measurement element to measureelectrogram amplitude at the tissue site, using the electrogrammeasurement element; and control circuitry operably coupled to thegenerator to initiate and terminate the application of ablation energyto the ablation element, wherein the control circuitry is operablycoupled to the electrogram amplitude measurement circuit and reduces thelevel of ablation energy to the ablation element responsive tooccurrence of a rapid drop in the electrogram amplitude measured by theelectrogram amplitude measurement circuitry following initiation ofapplication of ablation energy to the ablating element.
 13. The systemof claim 12 wherein the ablation element is an ablation electrode. 14.The system of claim 13 wherein the ablation electrode is an irrigatedablation electrode.
 15. The system of claim 13 wherein the ablationelectrode is employed as the electrogram measurement element.
 16. Thesystem of claim 12 wherein the electrogram measurement element is anelectrogram measurement electrode.
 17. The system of claim 12 whereinthe generator is an R-F generator.
 18. The system of claim 12 whereinthe ablation energy is R-F energy.
 19. The system of claim 12 whereinthe ablation energy is microwave radiation.
 20. The system of claim 12wherein the ablation energy is laser energy.
 21. The system of claim 12wherein the ablation energy is heat energy.
 22. The system of claim 12wherein the ablation energy is ultrasound energy.
 23. An ablationsystem, comprising: a generator for generating ablation energy; anablation device comprising an ablation element operably coupled to thegenerator and locatable adjacent a tissue site to be ablated, forapplying ablation energy to the tissue site; an electrogram measurementelement mounted to the ablation device so that the electrogrammeasurement element is adjacent the tissue site when the ablationelement is adjacent the tissue site; electrogram amplitude measurementcircuitry operably coupled to the electrogram measurement element tomeasure electrogram amplitude at the tissue site, using the electrogrammeasurement element; and control circuitry operably coupled to thegenerator to initiate and terminate the application of ablation energyto the ablation element, wherein the control circuitry is operablycoupled to the electrogram amplitude measurement circuit and terminatesthe application of ablation energy to the ablation element responsive tooccurrence of a drop in the electrogram amplitude measured by theelectrogram amplitude measurement circuitry following initiation ofapplication of ablation energy to the ablating element.
 24. The systemof claim 23 wherein the ablation element is an ablation electrode. 25.The system of claim 24 wherein the ablation electrode is an irrigatedablation electrode.
 26. The system of claim 24 wherein the ablationelectrode is employed as the electrogram measurement element.
 27. Thesystem of claim 23 wherein the electrogram measurement element is anelectrogram measurement electrode.
 28. The system of claim 23 whereinthe generator is an R-F generator.
 29. The system of claim 23 whereinthe ablation energy is R-F energy.
 30. The system of claim 23 whereinthe ablation energy is microwave radiation.
 31. The system of claim 23wherein the ablation energy is laser energy.
 32. The system of claim 23wherein the ablation energy is heat energy.
 33. The system of claim 23wherein the ablation energy is ultrasound energy.